Determining the association of SCAR marker ND5E and resistance to race 8 of Diplocarpon rosae Wolf in a diverse group of landscape roses

Roses are among the most popular flowering landscape shrubs in the world. Rose black spot disease, caused by the pathogen Diplocarpon rosae Wolf, is the most serious disease of outdoor-grown roses worldwide. Both race-specific and horizontal resistances to rose black spot have been documented in Rosa spp. and both resistance forms are valuable to pursue in the development of new resistant cultivars. Having robust markers linked to race-specific resistance genes is of great value to breeders for use in screening germplasm and working towards stacking multiple race-specific resistances in new cultivars. Currently the SCAR marker ND5E is the only known marker linked to Rdr3, a gene conferring resistance to race 8 of rose black spot. ND5E was identified in 'George Vancouver' and is estimated to reside 9.1 cM from the gene. Roses of diverse backgrounds (n=89) with known resistance to race 8 (28 resistant and 61 susceptible) were screened for ND5E (n≥3 reactions per cultivar). The marker (∼8 bp) was definitively recovered in 11 roses (9 resistant and 2 susceptible), and for many of the other roses faint bands of similar size were inconsistently produced in some of the amplifications. Although the low false positive rate for susceptible roses clearly possessing the marker is promising (3.2%), the low detection rate of ND5E among resistant roses (32.1%) suggests a lack of universality for this marker. Relatives of resistant roses possessing ND5E were subsequently screened for ND5E, and the presence of the marker was found in family lines. Efforts are underway to identify new markers that are more tightly linked to Rdr3.